Mather once noted that his life was "a continual conversation with heaven," but he spent agonizing hours convinced that he was damned and equal time in ecstasies that he was not. For a while, he feared he could not enter the ministry because of a speech impediment, and he considered becoming a physician; the subject of medicine was of lifelong interest to him. After a friend persuaded him "to oblige himself to a dilated Deliberation in speaking," he conquered his weakness and returned to religious studies. He preached his first sermon in his father's church in August 1680 and in October another from his grandfather John Cotton's pulpit. He was formally ordained in 1685 and became his father's colleague.

He devoted his life to praying, preaching, writing, and publishing and still followed his main purpose in life of doing good. His book, Bonifacius, or Essays to Do Good (1710), instructs others in humanitarian acts, some ideas being far ahead of his time: the schoolmaster to reward instead of punish his students, the physician to study the state of mind of his patient as a probable cause of illness. He established societies fo